There is the one issue which I fixed in LLVM, but it should be fixed in Zig as well, because the clobber list in Zig is typed and gives you false impression that adding x30 there is valid. But there is also another issue, x18 is a general purpose register outside of Darwin and Windows and needs to be marked as clobbered on other systems. And yes, look at the linked changes, the stdlib has inline assembly for the context switching.